Look-up table based logic macro-cells

ABSTRACT

A programmable look up table (LUT) structure that offers higher logic packing capacity over conventional LUT structures for programmable logic devices is disclosed. A programmable LUT structure comprising a first stage and one or more intermediate stages and a last stage, wherein at least one of said intermediate stages or the last stage further comprises: a primary input received in true and compliment logic levels, and an output; and two LUT values, said primary input coupling one of said LUT values to said output, wherein at least one of said LUT values further comprises: a secondary input and a configurable data value; and a programmable means to select either the secondary input or the data value as the LUT value.

This application is a continuation of application Ser. No. 10/743,894(now U.S. Pat. No. 7,019,557), filed on Dec. 24, 2003, which is relatedto application Ser. Nos. 10/267,483, 10/267,484 and 10/267,511 (now U.S.Pat. No. 6,747,478), all of which were filed on Oct. 8, 2002 and list asinventor Mr. R. U. Madurawe, the contents of which are incorporatedherein by reference.

This application is also related to application Ser. No. 10/413,808 (nowabandoned), Ser. No. 10/413,809 (now U.S. Pat. No. 6,856,030) and Ser.No. 10/413,810 (now U.S. Pat. No. 6,828,689), all of which were filedApr. 14, 2003 and list as inventor Mr. R. U. Madurawe, the contents ofwhich are incorporated herein by reference. This application is furtherrelated to application Ser. No. 10/691,013 filed Oct. 23, 2003, Ser. No.10/727,170 filed Dec. 4, 2003, Ser. No. 10/762,627 (now U.S. Pat. No.7,018,875) filed Jan. 23, 2004, Ser. No. 10/846,699 filed May, 17, 2004,Ser. No. 10/937,828 filed Sep. 10, 2004 and Ser. No. 10/988,396 filedNov. 15, 2004, all of which list as inventor Mr. R. U. Madurawe, thecontents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to look up table based macrocells forprogrammable logic applications.

Traditionally, application specific integrated circuit (ASIC) deviceshave been used in the integrated circuit (IC) industry to reduce cost,enhance performance or meet space constraints. The generic class of ASICdevices falls under a variety of sub classes such as Custom ASIC,Standard cell ASIC, Gate Array and Field Programmable Gate Array (FPGA)where the degree of user allowed customization varies. In thisdisclosure the word ASIC is used only in reference to Custom andStandard Cell ASICs where the designer has to incur the cost of a fullfabrication mask set. The term FPGA denotes an off the shelfprogrammable device with no fabrication mask costs, and Gate Arraydenotes a device with partial mask costs to the designer. The devicesFPGA include Programmable Logic Devices (PLD) and Complex ProgrammableLogic Devices (CPLD), while the devices Gate Array include LaserProgrammable Gate Arrays (LPGA), Mask Programmable Gate Arrays (MPGA)and a new class of devices known as Structured ASIC or StructuredArrays.

The design and fabrication of ASICs can be time consuming and expensive.The customization involves a lengthy design cycle during the productdefinition phase and high Non Recurring Engineering (NRE) costs duringmanufacturing phase. In the event of finding a logic error in the customor semi-custom ASIC during final test phase, the design and fabricationcycle has to be repeated. Such lengthy correction cycles furtheraggravate the time to market and engineering cost. As a result, ASICsserve only specific applications and are custom built for high volumeand low cost. The high cost of masks and unpredictable device life timeshipment volumes have caused ASIC design starts to fall precipitously inthe IC industry. ASICs offer no device for immediate designverification, no interactive design adjustment capability, and require afull mask set for fabrication.

Gate Array customizes pre-defined modular blocks at a reduced NRE costby designing the module connections with a software tool similar to thatin ASIC. The Gate Array has an array of non programmable (or moderatelyprogrammable) functional modules fabricated on a semiconductorsubstrate. To interconnect these modules to a user specification,multiple layers of wires are used during design synthesis. The level ofcustomization may be limited to a single metal layer, or single vialayer, or multiple metal layers, or multiple metals and via layers. Thegoal is to reduce the customization cost to the user, and provide thecustomized product faster. As a result, the customizable layers aredesigned to be the top most metal and via layers of a semiconductorfabrication process. This is an inconvenient location to customizewires. The customized transistors are located at the substrate level ofthe Silicon. All possible connections have to come up to the top levelmetal. The complexity of bringing up connections is a severe constraintfor these devices. Structured ASICs fall into larger module Gate Arrays.These devices have varying degrees of complexity in the structured celland varying degrees of complexity in the custom interconnection. Theabsence of Silicon for design verification and design optimizationresults in multiple spins and lengthy design iterations to the end user.The Gate Array evaluation phase is no different to that of an ASIC. Theadvantage over ASIC is in a lower upfront NRE cost for the fewercustomization layers, tools and labor, and the shorter time to receivethe finished product. Gate Arrays offer no device for immediate designverification, no interactive design adjustment capability, and require apartial mask set for fabrication. Compared to ASICs, Gate Arrays offer alower initial cost and a faster turn-around to debug the design. The endIC is more expensive compared to an ASIC.

In recent years there has been a move away from custom, semi-custom andGate Array ICs toward field programmable components whose function isdetermined not when the integrated circuit is fabricated, but by an enduser “in the field” prior to use. Off the shelf FPGA products greatlysimplify the design cycle and are fully customized by the user. Theseproducts offer user-friendly software to fit custom logic into thedevice through programmability, and the capability to tweak and optimizedesigns to improve Silicon performance. Provision of thisprogrammability is expensive in terms of Silicon real estate, butreduces design cycle time, time to solution (TTS) and upfront NRE costto the designer. FPGAs offer the advantages of low NRE costs, fastturnaround (designs can be placed and routed on an FPGA in typically afew minutes), and low risk since designs can be easily amended late inthe product design cycle. It is only for high volume production runsthat there is a cost benefit in using the other two approaches. Comparedto FPGA, an ASIC and Gate Array both have hard-wired logic connections,identified during the chip design phase. ASIC has no multiple logicchoices and both ASIC and most Gate Arrays have no configuration memoryto customize logic. This is a large chip area and a product cost savingfor these approaches to design. Smaller die sizes also lead to betterperformance. A full custom ASIC has customized logic functions whichtake less gate counts compared to Gate Arrays and FPGA configurations ofthe same functions. Thus, an ASIC is significantly smaller, faster,cheaper and more reliable than an equivalent gate-count FPGA. A GateArray is also smaller, faster and cheaper compared to an equivalentFPGA. The trade-off is between time-to-market (FPGA advantage) versuslow cost and better reliability (ASIC advantage). A Gate Array falls inthe middle with an improvement in the ASIC NRE cost at a moderatepenalty to product cost and performance. The cost of Silicon real estatefor programmability provided by the FPGA compared to ASIC and Gate Arraycontribute to a significant portion of the extra cost the user has tobear for customer re-configurability in logic functions.

In an FPGA, a complex logic design is broken down to smaller logicblocks and programmed into logic blocks provided in the FPGA. Logicblocks contain multiple smaller logic elements. Logic elementsfacilitates sequential and combinational logic design implementations.Combinational logic has no memory and outputs reflect a function solelyof present input states. Sequential logic is implemented by insertingmemory in the form of a flip-flop into the logic path to store pasthistory. Current FPGA architectures include transistor pairs, NAND or ORgates, multiplexers, look-up-tables (LUT) and AND-OR structures in abasic logic element. In a PLD the basic logic element is labeled amacro-cell. Hereafter the terminology logic element will include bothlogic elements and macro-cells. Granularity of an FPGA refers to logiccontent in the basic logic block. Partitioned smaller blocks of acomplex logic design are customized to fit into FPGA grain. Infine-grain architectures, one or a few small basic logic elements aregrouped to form a basic logic block, then enclosed in a routing matrixand replicated. A fine grain logic element may contain a 2-input MUX ora 2-input LUT and a register. These offer easy logic fitting at theexpense of complex routing. In course grain architectures, many largerlogic elements are combined into a basic logic block with local routing.A course grain logic element may include a 4-input LUT with a register,and a logic block may include as many as 4 to 8 logic elements. Thelarger logic block is then replicated with a global routing matrix.Larger logic blocks make the logic fitting difficult and the routingeasier. A challenge for FPGA architectures is to provide easy logicfitting (like fine grain) and maintain easy routing (like course grain).Course grain architectures are faster in logic operations and there isan increasing need in the IC industry to utilize larger logic blockswith multiple bigger LUT structures.

For sequential logic designs, the logic element may also includeflip-flops. A MUX based exemplary logic element described in Ref-1(Seals & Whapshott) is shown in FIG. 1A. The logic element has a builtin D-flip-flop 105 for sequential logic implementation. In addition,elements 101, 102 and 103 are 2:1 MUX's controlled by one input signalfor each MUX. Input S1 feeds into 101 and 102, while inputs S1 and S2feeds into OR gate 104, and the output from OR gate feeds into 103.Element 105 is the D-Flip-Flop receiving Preset, Clear and Clocksignals. One may very easily represent the programmable MUX structure inFIG. 1A as a 2-input LUT; where A, B, C & D are LUT values, and S1,(S2+S3) are LUT inputs. Ignoring the global Preset & Clear signals,eight inputs feed into the logic block, and one output leaves the logicblock. All 2-input, all 3-input and some 4-input variable functions arerealized in the logic block and latched to the D-Flip-Flop. Inputs andoutputs for the Logic Element or Logic Block are selected from theprogrammable Routing Matrix. An exemplary routing matrix containinglogic elements as described in Ref-1 is shown in FIG. 1B. Each logicelement 112 is as shown in FIG. 1A. The 8 inputs and 1 output from logicelement 112 in FIG. 1B are routed to 22 horizontal and 12 verticalinterconnect wires that have programmable via connections 110. Theseconnections 110 may be anti-fuses or pass-gate transistors controlled bySRAM memory elements. The user selects how the wires are connectedduring the design phase, and programs the connections in the field. FPGAarchitectures for various commercially available FPGA devices arediscussed in Ref-1 (Seals & Whapshott) and Ref-2 (Sharma).

Logic implementation in logic elements is achieved by converting a logicequation or a truth table to a gate realization. The gate leveldescription comprising elements and nets is also called a netlist. Theresulting logic gates are ported to LUT or MUX structure in the logicelement. An exemplary truth table and a plurality of transistor gaterealizations are shown in FIG. 2. In FIG. 2A, a truth table of 4 inputvariables, A, B, C & D is shown. By grouping the logic ones in thetable, the output function can be expressed as AND & OR functions ofinputs as shown by the logic equation in FIG. 2A. An exemplary MUXimplementation of the logic function is shown in FIG. 2B. The MUX has3-control variables A, B and C, and the fourth variable D together withD′ (not D), logic one and logic zero are used as inputs to the MUX. Theinputs can be hard-wired or provided as programmable options. The MUXcomprises a plurality of pass-gates 201. For a 3-variable hard-wiredMUX, only 14 pass-gates such as 201 are needed. This is a very efficientimplementation of hard-wired logic. Any 4-variable truth table can berealized by the 3-control variable MUX as shown in FIG. 2B by wiring theinput values accordingly. The inputs to a programmable MUX logic elementcan be provided as shown in FIG. 2C. There is considerable overhead tomake the MUX inputs user programmable. In FIG. 2C, two programmablememory bits such as 202 per input are configured to couple the desiredinput value to I₁. Combining the two figures in FIGS. 2B & 2C, one cansee that a 4-input programmable MUX utilizes 62 pass-gates such as 201and 16 memory bits such as 202. For 6T CMOS SRAM memory, each memory bitoccupies 4 NMOS gates and 2 PMOS gates. Hence a programmable 4-input MUXimplementation takes up 158 transistors. In anti-fuse technology, eachinput wire connection can be built into a programmable anti-fuse betweentwo metal lines. That requires only decoding transistors at the end ofwire segments to program the anti-fuse elements, thus saving Siliconarea. Hence a programmable MUX as shown in FIG. 2B is not popular forSRAM based FPGAs, whereas it is a logical choice for anti-fuse basedFPGAs.

AND/OR realization of the logic function in FIG. 2A is shown in FIG. 2D.There are five 3-input AND gates and one 5-input OR gate to generate therequired F output. In full CMOS implementation, each 3-input AND is 6transistors, while 5-input OR is 10 transistors. Hence the AND/OR gaterealization in FIG. 2D takes up 40 transistors. The Silicon area is alsoimpacted by the latch-up related N-Well rules that mandate certainspacing restrictions between NMOS and PMOS transistors. For thisexample, the hard-wire MUX implementation took less gates compared tothe hard-wire AND/OR gate implementation, while the programmable MUXtook a considerable overhead.

Commercially available FPGAs use 3-input and 4-input look up tables(LUT). The more popular 4-input LUT implementation of the truth table inFIG. 2A is shown in FIG. 2E. Any 4-input function can be implemented inFIG. 2E by setting the LUT values. In this disclosure, we will name thisa 4LUT, where the word input is dropped for convenience and the numberof inputs is pre-fixed to the word LUT. The 4LUT has 16 LUT values,which can be hard-wired or programmable. LUT and MUX construction oflogic elements are very similar and both are commercially used in FPGA &Gate Array products as shown in Ref-1 & Ref-2. There are 30 pass-gates(such as 201) in FIG. 2E for the hard-wire 4LUT. This 30 gate 4LUT islarger than a 14 gate hard-wire MUX, but smaller than the 40 gatehard-wire AND/OR logic implementation. The 16 LUT values in the 4LUTdetermine the LUT function. Using 16 programmable registers such as 202for these inputs allows the 4LUT to be user programmable. The 16 memoryelements, in both programmable MUX and LUT options, utilize 96 extratransistors when implemented in 6T CMOS SRAM. Hence the programmable4LUT with 126 transistors is more economical compared to theprogrammable MUX option with 158 transistors. Thus LUT logic isextensively used in SRAM based FPGAs while MUX logic is used inanti-fuse based FPGAs and Gate Arrays.

FPGA and Gate Array architectures are discussed in Carter U.S. Pat. No.4,706,216, Freemann U.S. Pat. No. 4,870,302, ElGamal et al. U.S. Pat.No. 4,873,459, Freemann et al. U.S. Pat. Nos. 5,488,316 & 5,343,406,Trimberger et al. US Pat. No. 5,844,422, Cliff et al. U.S. Pat. No.6,134,173, Wittig et al. U.S. Pat. No. 6,208,163, Or-Bach U.S.2001/003428, Mendel U.S. Pat. No. 6,275,065, Lee et al. US 2001/0048320,Or-Bach U.S. Pat. No. 6,331,789, Young et al. U.S. Pat. No. 6,448,808,Sueyoshi et al. US 2003/0001615, Agrawal et al. U.S. 2002/0186044,Sugibayashi et al. US Pat. No. 6,515,511 and Pugh et al. US2003/0085733. These patents disclose programmable MUX and programmableLUT structures to build logic elements that are user configurable. Inall cases a routing block is used to provide inputs and outputs forthese logic elements, while the logic element is programmed to perform aspecific logic function. The routing-block is a hard-wire connection forGate Array and Structured ASIC devices. Within a logic element, each LUTis hard-wired to a specific size, said size determined by the number ofLUT inputs. This LUT is the smallest building block in the logic elementand cannot be sub-divided. As an example, a smaller 2-input logicfunction would occupy a 4LUT, if that is the smallest element available.That leads to Silicon utilization inefficiency. Within a logic block,multiple logic elements are grouped together in a pre-defined manner.The size of the logic block determines the granularity. As manufacturinggeometries shrink, the FPGA granularity gets larger, the LUT sizeincreases and the number of LUTs per logic block has to increase. Havinga large fixed LUT in the logic element further aggravates the Siliconutilization efficiency and is not flexible for next generation FPGAdesigns.

As the LUT structure gets large, the logic porting becomes moredifficult and Silicon utilization gets more inefficient. To illustrateLUT utilization efficiency, in FIG. 3 we provide the pass-gateconstruction required to build 1LUT, 2LUT, 3LUT, 4LUT and 5LUT logicelements. FIG. 3A shows a 1LUT comprising of two pass-gates 301 & 302,two LUT values contained in two programmable registers 303 & 304 and oneinput variable “A” in true and compliment. A 1LUT is simply a 2:1 MUXselecting one of two register values. Any 1-input function such as 2:1MUX, Logic-1, Logic-0, TRUE and INVERT can be realized by this 1LUT byprogramming the two LUT values. Signal A allows the LUT values in either303 or 304 to reach output F. There is a time delay for this to occur.That is a characteristic 1LUT delay time, which is optimized by sizingthe transistors 301 and 302 as needed. Faster time requires widertransistors. The symbol for 1LUT is shown in FIG. 3B, and this symbol isused to illustrate higher LUT constructions in FIG. 3C thru FIG. 3F.

A 2LUT is shown in FIG. 3C that can realize any 2-input function such asAND, NAND, OR, NOR, XOR among others. As shown in FIG. 3C, the 2LUT canbe constructed by hard-wiring three 1LUTs 311, 312 & 313 as shown. Thisis termed a LUT cone or a LUT tree and comprises two stages. First stagehas 1LUT 311 and 312 sharing a common input, while second stage has 1LUT313. Only the 1LUTs in the first stage 311 and 312 have LUT values. LUToutputs from first stage are fed as LUT values to second stage. Theseare hard-wire connections. In FIG. 3C, 1LUT outputs from 311 and 312 arefed as LUT values to 1LUT 313. A 2LUT delay comprises the time taken fora LUT value in the first stage to reach F. There are now two pass-gatesin series, and this delay is larger than for a 1LUT. Thus the pass-gatesneed to be wider to reduce the LUT delay. That increase in area and slowdown in performance hurt LUT logic trees. Similarly, 3LUT, 4LUT and 5LUTconstructions with 1LUTs are shown in FIG. 3D, FIG. 3E and FIG. 3Frespectively. Those pass-gates have to be even wider to improve LUTdelays. The 5LUT in FIG. 3F has 16 1LUTs in the first stage, 8 1LUTs inthe second stage, 4 1LUTs in the third stage, 2 1LUTs in the fourthstage and one 1LUT in the final fifth stage. A total of 31 1LUTs areused in FIG. 3F for the 5LUT construction. A K-LUT cone or a K-LUT treehas K-input variables, K-stages and 2^(K) LUT values to realize aK-input function. Each stage has one common input variable. 2^((K−1))outputs from first stage feed as LUT values into second stage.Consecutive LUT value reduction continues until the last stage, whenonly 2 LUT values feed the last stage, and one LUT output is obtained.The equivalent 1LUTs required to build a K-LUT is tabulated in FIG. 3G,and is shown to grow as (2^(K)−1). Logic porting to K-LUT is discussedby Ahmed et al. (Ref-3) for multiple K values. They have looked atporting 20 benchmark logic designs into varying LUT sizes: 1LUT, 2LUT,3LUT, 4LUT, 5LUT, 6LUT and 7LUT. The geometric average number of K-LUTsrequired for porting 20 designs, as shown in FIG. 10 in Ref-2, istabulated in the first 2 columns of FIG. 4. As can be seen, as the sizeof the K-LUT increases, the total number of K-LUTs required to fit anaverage design decreases. In addition, FIG. 4 also lists the equivalent1LUT per K-LUT (from FIG. 3G) in column 3, and calculates the equivalent1LUTs required for the design in column 4. Column 4 values are obtainedby multiplying values in column 2 by values in column 3. In FIG. 4, eachrow represents how many K-LUTs are required for an average design, andan equivalent 1LUT calculation as a measure of Silicon utilization. 2LUTimplementation in row-1 needs only 12900 1LUTs, while the 7LUTimplementation in row-6 needs 177800 1LUTs for the same design. Thelatter 7LUT has only 7.3% Silicon utilization efficiency compared to theformer 2LUT. From row-3, commercially available FPGAs with 4LUTs areseen only 36.1% efficient compared to 2LUTs at fitting logic. As the LUTsize gets larger, clearly a more efficient LUT circuit is needed toimprove Silicon utilization in LUT based logic elements.

LUT based logic elements are used in conjunction with programmable pointto point connections. Four exemplary methods of programmable point topoint connections, synonymous with programmable switches, between node Aand node B are shown in FIG. 5. A configuration circuit to program theconnection is not shown in FIG. 5. All the patents listed under FPGAarchitectures use one or more of these basic programmable connections.In FIG. 5A, a conductive fuse link 510 connects A to B. It is normallyconnected, and passage of a high current or exposure to a laser beamwill blow the conductor open. In FIG. 5B, a capacitive anti-fuse element520 disconnects A from B. It is normally open, and passage of a highcurrent will pop the insulator shorting the two terminals. Fuse andanti-fuse are both one time programmable due to the non-reversiblenature of the change. In FIG. 5C, a pass-gate device 530 connects A toB. The gate signal S₀ determines the nature of the connection, on oroff. This is a non destructive change. The gate signal is generated bymanipulating logic signals, or by configuration circuits that includememory. The choice of memory varies from user to user. In FIG. 5D, afloating-pass-gate device 540 connects A to B. Control gate signal S₀couples a portion of that to floating gate. Electrons trapped in thefloating gate determines an on or off state for the connection.Hot-electrons and Fowler-Nordheim tunneling are two mechanisms forinjecting charge to floating-gates. When high quality insulatorsencapsulate the floating gate, trapped charge stays for over 10 years.These provide non-volatile memory. EPROM, EEPROM and Flash memory employfloating-gates and are non-volatile. Anti-fuse and SRAM basedarchitectures are widely used in commercial FPGA's, while EPROM, EEPROM,anti-fuse and fuse links are widely used in commercial PLD's. VolatileSRAM memory needs no high programming voltages, is freely available inevery logic process, is compatible with standard CMOS SRAM memory, lendsto process and voltage scaling and has become the de-facto choice formodern day very large FPGA device construction.

All commercially available high density FPGA's use SRAM memory elements.A volatile six transistor SRAM based configuration circuit is shown inFIG. 6A. The SRAM memory element can be any one of 6-transistor,5-transistor, full CMOS, R-load or TFT PMOS load based cells to name afew. Two inverters 603 and 604 connected back to back forms the memoryelement. This memory element is a latch providing complementary outputsS₀ and S₀′. The latch can be constructed as full CMOS, R-load, PMOS loador any other. Power and ground terminals for the inverters are not shownin FIG. 6A. Access NMOS transistors 601 and 602, and access wires GA,GB, BL and BS provide the means to configure the memory element.Applying zero and one on BL and BS respectively, and raising GA and GBhigh enables writing zero into device 601 and one into device 602. Theoutput S₀ delivers a logic one. Applying one and zero on BL and BSrespectively, and raising GA and GB high enables writing one into device601 and zero into device 602. The output S₀ delivers a logic zero. TheSRAM construction may allow applying only a zero signal at BL or BS towrite data into the latch. The SRAM cell may have only one accesstransistor 601 or 602. The SRAM latch will hold the data state as longas power is on. When the power is turned off, the SRAM bit needs to berestored to its previous state from an outside permanent memory. In theliterature for programmable logic, this second non-volatile memory isalso called configuration memory. Upon power up, an external or aninternal CPU loads the external configuration memory to internalconfiguration memory locations. All of FPGA functionality is controlledby the internal configuration memory. The SRAM configuration circuit inFIG. 6A controlling logic pass-gate is illustrated in FIG. 6B. Element650 represents the configuration circuit. The S₀ output directly drivenby the memory element shown in FIG. 6A drives the pass-gate 610 gateelectrode. In addition to S₀ output and the memory cell, power, ground,data-in and write-enable signals in 650 constitutes the SRAMconfiguration circuit. Write enable circuitry includes GA, GB, BL, BSsignals shown in FIG. 6A.

As discussed earlier, providing programmability is a very severetransistor and cost penalty compared to hard-wired Gate Array or ASICimplementation of identical logic. A significant factor in the penaltycomes from the 6-transistors required for the configuration circuits.The natural conclusion is to minimize the number of configurable bitsused in the programmable logic element. This mandates constructing ahard-wired larger 6LUT or a bigger LUT for next generation FPGAs. Wehave shown that Silicon utilization is severely impacted with this movetowards larger LUT structures in logic elements. What is desirable is tohave an economical and flexible LUT macro-cell, or a macro-LUT circuit.This LUT macro-cell should efficiently implement logic functions. Bothlarge logic functions that port to one big LUT and small logic functionsthat port to multiple smaller LUTs should fit easily into a LUTmacro-cell. Furthermore, LUT logic packing should maximize Siliconutilization to keep programmable logic cost reasonable with otherhard-wired IC manufacturing choices. The user should be able to take asynthesized netlist from an ASIC flow, typically comprising smallerlogic blocks, convert this netlist to fit in the FPGA granularity, placeand route logic economically and efficiently. This would make use ofexisting third party ASIC tools at the front-end logic design andstreamline tool flow for FPGA place & routing.

For an emulation device, the cost of programmability is not the primaryconcern if such a device provides a migration path to a lower cost.Today an FPGA migration to a Gate Array requires a new design to ensuretiming closure. A desirable migration path is to keep the timing of theoriginal FPGA design intact. That would avoid valuable re-engineeringtime, opportunity costs and time to solution (TTS). Such a conversionshould occur in the same base die to avoid Silicon and systemre-qualification costs and implementation delays. Such a conversionshould also realize an end product that is competitive with anequivalent standard cell ASIC or a Gate Array product in cost andperformance. Such an FPGA device will also target applications that arecost sensitive, have short life cycles and demand high volumes.

SUMMARY

In one aspect, a programmable look up table (LUT) circuit for anintegrated circuit comprises: one or more secondary inputs; and one ormore configurable logic states; and two or more LUT values; and aprogrammable means to select a LUT value from a secondary input or aconfigurable logic state.

Implementations of the above aspect may include one or more of thefollowing. A semiconductor integrated circuit comprises an array ofprogrammable modules. Each module may use one or more LUT or MUX basedlogic elements. A programmable interconnect structure may be used tointerconnect these programmable modules in an FPGA device. A logicdesign may be specified by the user in VHDL or Verilog design inputlanguage and synthesized to a gate-level netlist description. Thissynthesized netlist is ported into logic blocks and connected by therouting block in the FPGA. Each large LUT in a module may be comprisedof a smaller 1-input LUT (1LUT) cone, known also as a 1LUT tree. ALarger LUT may be comprised of smaller 2LUT, or 3LUT trees. A smallerLUT provides added flexibility in fitting logic. A smaller LUT providesat least one LUT value to be selected from either a programmableregister or from an input. The input may be an output of a previouslygenerated logic function, or an external input. The registers may beuser configurable to logic zero and logic one states. The larger LUT andsmaller LUT may comprise a programmable switch to connect two points.Most common switch is a pass-gate device. A pass-gate is an NMOStransistor, or a PMOS transistor or a CMOS transistor pair that canelectrically connect two points. Other methods of connecting two pointsinclude fuse links and anti-fuse capacitors, among others. Programmingthese devices include forming one of either a conducting path or anon-conducting path in the connecting device. These pass-gates may befabricated in a first module layer, said module comprising a Siliconsubstrate layer.

The LUT circuits may include digital circuits consisting of CMOStransistors forming AND, NAND, INVERT, OR, NOR and pass-gate type logiccircuits. Configuration circuits are used to change LUT values,functionality and connectivity. Configuration circuits have memoryelements and access circuitry to change stored memory data. Memoryelements can be RAM or ROM. Each memory element can be a transistor or adiode or a group of electronic devices. The memory elements can be madeof CMOS devices, capacitors, diodes, resistors, wires and otherelectronic components. The memory elements can be made of thin filmdevices such as thin film transistors (TFT), thin-film capacitors andthin-film diodes. The memory element can be selected from the groupconsisting of volatile and non volatile memory elements. The memoryelement can also be selected from the group comprising fuses, antifuses,SRAM cells, DRAM cells, optical cells, metal optional links, EPROMs,EEPROMs, flash, magnetic, electro-chemical and ferro-electric elements.One or more redundant memory elements can be provided for controllingthe same circuit block. The memory element can generate an output signalto control pass-gate logic. Memory element may generate a signal that isused to derive a control signal to control pass-gate logic. The controlsignal is coupled to MUX or Look-Up-Table (LUT) logic element.

LUT circuits are fabricated using a basic logic process used to buildCMOS transistors. These transistors are formed on a P-type, N-type, epior SOI substrate wafer. Configuration circuits, including configurationmemory, constructed on same Silicon substrate take up a large Siliconfoot print. That adds to the cost of programmable LUT circuits comparedto similar functionality custom wire circuits. A 3-dimensionalintegration of configuration circuits described in incorporatedreferences provides a significant cost reduction in programmability. Theconfiguration circuits may be constructed after a first contact layer isformed or above one or more metal layers. The programmable LUT may beconstructed as logic circuits and configuration circuits. Theconfiguration circuits may be formed vertically above the logic circuitsby inserting a thin-film transistor (TFT) module. The TFT module mayinclude one or more metal layers for local interconnect between TFTtransistors. The TFT module may include salicided poly-Silicon localinterconnect lines and thin film memory elements. The thin-film modulemay comprise thin-film RAM elements. The thin-film memory outputs may bedirectly coupled to gate electrodes of LUT pass-gates to provideprogrammability. Contact or via thru-holes may be used to connect TFTmodule to underneath layers. The thru-holes may be filled withTitanium-Tungsten, Tungsten, Tungsten Silicide, or some other refractorymetal. The thru-holes may contain Nickel to assist Metal Induced LaserCrystallization (MILC) in subsequent processing. Memory elements mayinclude TFT transistors, capacitors and diodes. Metal layers above theTFT layers may be used for all other routing. This simple verticallyintegrated pass-gate switch and configuration circuit reducesprogrammable LUT cost.

In a second aspect, a programmable look up table circuit for anintegrated circuit comprises: M primary inputs, wherein M is an integervalue greater than or equal to one, and each said M inputs received intrue and compliment logic levels; and 2^(M) secondary inputs; and 2^(M)configurable logic states, each said state comprising a logic zero and alogic one; and 2^(M) LUT values; and a programmable means to select eachof said LUT values from a secondary input or a configurable logic state.

Implementations of the above aspect may include one or more of thefollowing. A larger N-LUT is constructed with all equal size smallerK-LUTs. A larger N-LUT is constructed with unequal sized smaller K-LUTs.Each smaller K-LUT is constructed as a 1LUT, 2LUT, 3LUT up to (N−1)-LUT.The N-LUT is constructed as a K-LUT tree. Each stage in the N-LUT treecomprises a plurality of K-LUTs. Each K-LUT has one output. Larger N-LUThas one or more outputs comprising a plurality of smaller K-LUT outputs.Each K-LUT is also constructed as a 1LUTs tree. All primary K-LUTs (thefirst set of K-LUTs) in the N-LUT tree may have only configurable logicstates for LUT values. All primary K-LUTs may a have a LUT valueselected from an input and a configurable logic state. Said input maycomprise an external input, a feed-back signal, a memory output or acontrol signal. Secondary K-LUT in the N-LUT tree provides aprogrammable connection between previous K-LUT outputs and configurablelogic states. This hierarchical K-LUT arrangement is termed herein a LUTmacrocell circuit. A LUT macrocell provides programmability to implementlogic as one large N-LUT or as multiple smaller K-LUTs. Such division inlogic implementation allows more logic to fit in a single LUT macrocell.It provides course-grain architecture with fine-grain logic fittingcapability. More logic fitting improves Silicon utilization. In oneembodiment, the smaller K-LUTs are implemented as 1LUTs. In a secondembodiment the smaller K-LUTs are implemented as 2LUTs. In yet anotherembodiment the smaller K-LUTs are implemented as 3LUTs. A 1LUT in thefirst stage of a secondary K-LUT is used to combine two outputs fromprior K-LUTs.

In a third aspect, a programmable macro look up table (macro-LUT)circuit for an integrated circuit, comprises: a plurality of LUTcircuits, each of said LUT circuits comprising a LUT output, at leastone LUT input, and at least two LUT values; and a programmable means ofselecting LUT inputs to at least one of said LUT circuits from one ormore other LUT circuit outputs and external inputs, and selecting LUTvalues to at least one of said LUT circuits from one or more other LUTcircuit outputs and configurable logic states, said programmable meansfurther comprised of two selectable manufacturing configurations,wherein: in a first selectable configuration, a random access memorycircuit (RAM) is formed, said memory circuit further comprisingconfigurable thin-film memory elements; in a second selectableconfiguration, a hard-wire read only memory circuit (ROM) is formed inlieu of said RAM, said ROM duplicating one RAM pattern in the firstselectable option.

Implementations of the above aspect may include one or more of thefollowing. A programmable macro-LUT is used for a user to customizelogic in an FPGA. This programmability is provided to the user in an offthe shelf FPGA product. There is no waiting and time lost to portsynthesized logic design into a macro-LUT circuit. This reduces time tosolution (TTS) by 6 months to over a year. The macro-LUT can besub-divided into smaller LUT circuits. Each smaller LUT is comprised of1LUTs. A portion of macro-LUT inputs and LUT values are selected by aprogrammable method. This allows prior LUT output logic manipulation.Macro-LUT inputs are selected from external inputs or other LUT outputs.LUT values are selected from external inputs, other LUT outputs orconfigurable logic states. Macro-LUT is very flexible in fitting onelarge logic block or many smaller logic blocks. Macro-LUT improvesSilicon utilization. Macro-LUT improves run-times of a software toolthat ports logic designs into FPGA. Macro-LUT improves routability. TheMacro-LUT is constructed with RAM and ROM options.

Implementations of the above aspect may include one or more of thefollowing. A programmable method includes customizing programmable LUTchoices. This may be done by the user, wherein the macro-LUT comprisesconfiguration circuits, said circuits including memory elements.Configuration circuits may be constructed in a second module,substantially above a first module comprising LUT pass-gate transistors.Configuration memory is built as Random Access Memory (RAM). User maycustomize the RAM module to program the LUT connections. The RAMcircuitry may be confined to a thin-film transistor (TFT) layer in thesecond module. This TFT module may be inserted to a logic process.Manufacturing cost of TFT layers add extra cost to the finished product.This cost makes a programmable LUT less attractive to a user who hascompleted the programming selection. Once the programming is finalizedby the user, the LUT connections and the RAM bit pattern is fixed formost designs during product life cycle. Programmability in the LUTcircuit is no longer needed and no longer valuable to the user. The usermay convert the design to a lower cost hard-wire ROM circuit. Theprogrammed LUT choices are mapped from RAM to ROM. RAM outputs at logicone are mapped to ROM wires connected to power. RAM outputs at logiczero are mapped to ROM wires connected to ground. This may be done witha single metal mask in lieu of all of the TFT layers. Such anelimination of processing layers reduces the cost of the ROM version. Afirst module with macro-LUT transistors does not change by thisconversion. A third module may exist above the second module to completeinterconnect for functionality of the end device. The third module alsodoes not change with the second module option. A timing characteristiccomprising signal delay for LUT values to reach LUT output is notchanged by the memory option. The propagation delays and critical pathtiming in the FPGA may be substantially identical between the two secondmodule options. The TFT layers may allow a higher power supply voltagefor the user to emulate performance at reduced pass-gate resistances.Such emulations may predict potential performance improvements for TFTpass-gates and hard-wired connected options. Duplicated ROM pattern maybe done with a customized thru-hole mask. Customization may be done witha thru-hole and a metal mask or a plurality of thru-hole and metalmasks. Hard wire pattern may also improve reliability and reduce defectdensity of the final product. The ROM pattern provides a cost economicalfinal macro-LUT circuit to the user at a very low NRE cost. The totalsolution provides a programmable and customized solution to the user.

Implementations of the above aspect may further include one or more ofthe following. The programmable LUT circuit comprises a RAM element thatcan be selected from the group consisting of volatile or non volatilememory elements. The memory can be implemented using a TFT processtechnology that contains one or more of Fuses, Anti-fuses, DRAM, EPROM,EEPROM, Flash, Ferro-Electric, optical, magnetic, electro-chemical andSRAM elements. Configuration circuits may include thin film elementssuch as diodes, transistors, resistors and capacitors. The processimplementation is possible with any memory technology where theprogrammable element is vertically integrated in a removable module. Themanufacturing options include a conductive ROM pattern in lieu of memorycircuits to control the logic in LUT circuits. Multiple memory bitsexist to customize wire connections inside macro-LUTs, inside a logicblock and between logic blocks. Each RAM bit pattern has a correspondingunique ROM pattern to duplicate the same functionality.

The programmable LUT structures described constitutes fabricating a VLSIIC product. The IC product is re-programmable in its initial stage withturnkey conversion to a one mask customized ASIC. The IC has the endASIC cost structure and initial FPGA re-programmability. The IC productoffering occurs in two phases: the first phase is a generic FPGA thathas re-programmability contained in a programmable LUT and programmablewire circuit, and a second phase is an ASIC that has the entireprogrammable module replaced by one or two customized hard-wire masks.Both FPGA version and turnkey custom ASIC has the same base die. Nore-qualification is required by the conversion. The verticallyintegrated programmable module does not consume valuable Silicon realestate of a base die. Furthermore, the design and layout of theseproduct families adhere to removable module concept: ensuring thefunctionality and timing of the product in its FPGA and ASIC canonicals.These IC products can replace existing PLD's, CPLD's, FPGA's, GateArrays, Structured ASIC's and Standard Cell ASIC's. An easy turnkeycustomization of an end ASIC from an original smaller cheaper and fasterprogrammable structured array device would greatly enhance time tomarket, performance, product reliability and solution cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an exemplary MUX or LUT based logic element.

FIG. 1B shows an exemplary programmable wire structure utilizing a logicelement.

FIG. 2A shows a truth table for a four variable function and the logicequation.

FIG. 2B shows a 3-control-variable MUX realization of the function shownin FIG. 2A.

FIG. 2C shows a MUX input connection for a programmable version of MUXin FIG. 2B.

FIG. 2D shows an AND/OR gate realization of the function shown in FIG.2A.

FIG. 2E shows a 4-input LUT realization of the function shown in FIG.2A.

FIG. 3A shows an exemplary one input LUT (1LUT).

FIG. 3B shows the symbol for 1LUT in FIG. 3A that is used in rest ofFIG. 3.

FIG. 3C–FIG. 3F shows exemplary 2LUT, 3LUT, 4LUT and 5LUT respectively.

FIG. 3G shows the number of 1LUTs needed to construct a K-LUT, where Kis an integer from 1 to 7.

FIG. 4 shows Silicon utilization efficiency with K-LUTs, extracted fromFIG. 10 in Ref-3.

FIG. 5A shows an exemplary fuse link point to point connection.

FIG. 5B shows an exemplary anti-fuse point to point connection.

FIG. 5C shows an exemplary pass-gate point to point connection.

FIG. 5D shows an exemplary floating-pass-gate point to point connection.

FIG. 6A shows an exemplary configuration circuit for a 6T SRAM element.

FIG. 6B shows an exemplary programmable pass-gate switch with SRAMmemory.

FIG. 7 shows an anti-fuse based configuration circuit.

FIG. 8A shows a first embodiment of a floating gate configurationcircuit.

FIG. 8B shows a second embodiment of a floating gate configurationcircuit.

FIG. 9 shows a modular construction of a LUT circuit with removable TFTlayers.

FIG. 10.1–10.7 shows process cross-sections of one embodiment tointegrate thin-film transistors into a logic process in accordance withthe current invention.

FIG. 11A shows a novel programmable 1-input LUT (1LUT).

FIG. 11B shows the 1LUT in FIG. 11A with a programmable MUX to selectLUT values.

FIG. 11C shows the 1LUT block diagram in FIG. 11A with a configurableLUT value.

FIG. 11D shows the 1LUT block diagram in FIG. 11A with two configurableLUT values.

FIG. 12A shows a second embodiment of a novel programmable 1LUT.

FIG. 12B shows a third embodiment of a novel programmable 1LUT.

FIG. 13A shows a fourth embodiment of a novel programmable 1LUT.

FIG. 13B shows a fifth embodiment of a novel programmable 1LUT.

FIG. 14 shows a novel programmable 2LUT macro-cell.

FIG. 15 shows a novel programmable 3LUT macro-cell.

FIG. 16A shows a first embodiment of a novel programmable 4LUTmacro-cell.

FIG. 16B shows a second embodiment of a novel programmable 4LUTmacro-cell.

FIG. 17A shows a first embodiment of a novel programmable 3LUT.

FIG. 17B shows a second embodiment of a novel programmable 3LUT.

FIG. 18A shows a truth table and logic equation of an example.

FIG. 18B shows a 2LUT gate realization of the logic function in FIG.18A.

FIG. 18C shows a 4LUT gate realization of the logic function in FIG.18B.

FIG. 18D shows a programmable 4LUT gate realization of logic function inFIG. 18B.

DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe present invention.

Definitions: The terms wafer and substrate used in the followingdescription include any structure having an exposed surface with whichto form the integrated circuit (IC) structure of the invention. The termsubstrate is understood to include semiconductor wafers. The termsubstrate is also used to refer to semiconductor structures duringprocessing, and may include other layers that have been fabricatedthereupon. Both wafer and substrate include doped and undopedsemiconductors, epitaxial semiconductor layers supported by a basesemiconductor or insulator, SOI material as well as other semiconductorstructures well known to one skilled in the art. The term conductor isunderstood to include semiconductors, and the term insulator is definedto include any material that is less electrically conductive than thematerials referred to as conductors.

The term module layer includes a structure that is fabricated using aseries of predetermined process steps. The boundary of the structure isdefined by a first step, one or more intermediate steps, and a finalstep. The resulting structure is formed on a substrate.

The term pass-gate refers to a structure that can pass a signal when on,and blocks signal passage when off. A pass-gate connects two points whenon, and disconnects two points when off. A pass-gate can be afloating-gate transistor, an NMOS transistor, a PMOS transistor or aCMOS transistor pair. A pass-gate can be an electrolytic cell. In oneembodiment, the gate electrode of pass-gate determines the state of theconnection. A CMOS pass-gate requires complementary signals coupled toNMOS and PMOS gate electrodes. A control logic signal is connected togate electrode of a pass-gate for programmable logic. In anotherembodiment a gate-electrode may used to configure a pass-gate betweenthe on and off conditions. A programming voltage may be applied toconfigure the pass-gate to a pre-established state. The on, offconditions may be induced by altering the properties of the materialsused to construct the pass-gate element.

The term configuration circuit includes one or more configurableelements and connections that can be programmed to control one or morecircuit blocks in accordance with a predetermined user-desiredfunctionality. The configuration circuit includes the memory element andthe access circuitry, herewith called memory circuitry, to modify saidmemory element. Configuration circuit does not include the logicpass-gate controlled by said memory element. In one embodiment, theconfiguration circuit includes a plurality of RAM circuits to storeinstructions to configure an FPGA. In another embodiment, theconfiguration circuit includes a first selectable configuration where aplurality of RAM circuits is formed to store instructions to control oneor more circuit blocks. The configuration circuits include a secondselectable configuration with a predetermined ROM conductive patternformed in lieu of the RAM circuit to control substantially the samecircuit blocks. The memory circuit includes elements such as diode,transistor, resistor, capacitor, metal link, wires, among others. Thememory circuit also includes thin film elements. In yet anotherembodiment, the configuration circuits include a predeterminedconductive pattern, contact, via, resistor, capacitor or other suitablecircuits formed in lieu of the memory circuit to control substantiallythe same circuit blocks.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizontaldirection as defined above. Prepositions, such as “on”, “side”,“higher”, “lower”, “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

The term look up table structure, LUT structure, LUT and LUT circuit areused for a look up table logic circuit. A LUT structure includes a LUTcone or a LUT tree. A LUT structure includes a 2:1 multiplexer. A LUTstructure comprises a plurality of 2:1 multiplexer elements. A LUTstructure includes NMOS, PMOS and CMOS transistors, and other circuitelements required to construct the LUT circuit. A LUT structure includesthin-film transistors, and 2D/3D arrangements of circuit elements. A LUTstructure includes a pass-gate. A LUT structure includes inputs, outputsand data values. A LUT structure includes LUT stages. A LUT stage isdefined as one or more 2:1 multiplexer elements in a LUT structuresharing a primary input. Primary inputs may be received by the LUTstructure in true and compliment forms. The term K-LUT refers to a lookup table comprising K inputs. A LUT value is defined as the look uptable value read by the inputs. True and compliment nature of theprimary signal will allow one of two LUT values in the 2:1 multiplexerelement input to couple to the output. A K-LUT comprises 2^(K) LUTvalues, and at least one output. For a given combination of K-inputvalues, a LUT value is received at said at least one LUT output. WhenK=1, the 1-LUT is a 2:1 multiplexer. A LUT stage comprising a pluralityof 2:1 multiplexers also comprises a plurality of outputs. The terms LUTtree and LUT cone refers to the construction of a LUT structure, whereinthere is a gradual decrease in the number of multiplexers in each stage.A first of the K-inputs is common to all the multiplexers in a firststage, a second of the K-inputs is common to all the multiplexers in asecond stage and so on until the last LUT stage is reached in a hardwired K-LUT tree. The LUT stages in between the first LUT stage and lastLUT stage are defined as intermediate LUT stages.

Programmable LUTs use point to point connections that utilizeprogrammable pass-gate logic as shown in FIG. 6A and FIG. 6B. Multipleinputs (node A) can be connected to multiple outputs (node B) with aplurality of pass-gate logic elements. The SRAM base connection shown inFIG. 6 may have pass-gate 610 as a PMOS or an NMOS transistor. NMOS ispreferred due to its higher conduction. The voltage S₀ on NMOStransistor 610 gate electrode determines an ON or OFF connection. Thatlogic level is generated by a configuration circuit 650 coupled to thegate of NMOS transistor 610. The pass-gate logic connection requires theconfiguration circuitry to generate signal S₀ with sufficient voltagelevels to ensure off and on conditions. For an NMOS pass-gate, S₀ havinga logic level one completes the point to point connection, while a logiclevel zero keeps them disconnected. In addition to using only an NMOSgate, a PMOS gate could also be used in parallel to make the connection.The configuration circuit 650 needs to then provide complementaryoutputs (S₀ and S₀′) to drive NMOS and PMOS gates in the connection.Configuration circuit 650 contains a memory element. Most CMOS SRAMmemory delivers complementary outputs. This memory element can beconfigured by the user to select the polarity of S₀, thereby selectingthe status of the connection. The memory element can be volatile ornon-volatile. In volatile memory, it could be DRAM, SRAM, Optical or anyother type of a memory device that can output a valid signal S₀. Innon-volatile memory it could be fuse, anti-fuse, EPROM, EEPROM, Flash,Ferro-Electric, Magnetic or any other kind of memory device that canoutput a valid signal S₀. The output S₀ can be a direct output coupledto the memory element, or a derived output in the configurationcircuitry. An inverter can be used to restore S₀ signal level to fullrail voltage levels. The SRAM in configuration circuit 650 can beoperated at an elevated Vcc level to output an elevated S₀ voltagelevel. This is especially feasible when the SRAM is built in a separateTFT module. Other configuration circuits to generate a valid S₀ signalare discussed next.

An anti-fuse based configuration circuit to use with this invention isshown next in FIG. 7. Configuration circuit 650 in FIG. 6B can bereplaced with the anti-fuse circuit shown in FIG. 7. In FIG. 7, outputlevel S₀ is generated from node X which is coupled to signals VA and VBvia two anti-fuses 750 and 760 respectively. Node X is connected to aprogramming access transistor 770 controlled by gate signal GA and drainsignal BL. A very high programming voltage is needed to blow theanti-fuse capacitor. This programming voltage level is determined by theanti-fuse properties, including the dielectric thickness. Assertingsignal VA very high, VB low (typically ground), BL low and GA high (Vccto pass the ground signal) provides a current path from VA to BL throughthe on transistor 770. A high voltage is applied across anti-fuse 750 topop the dielectric and short the terminals. Similarly anti-fuse 760 canbe programmed by selecting VA low, VB very high, BL low and GA high.Only one of the two anti-fuses is blown to form a short. When theprogramming is done, BL and GA are returned to zero, isolating node Xfrom the programming path. VA=Vss (ground) and VB=Vcc (power, orelevated Vcc) is applied to the two signal lines. Depending on the blownfuse, signal S₀ will generate a logic low or a logic high signal. Thisis a one time programmable memory device. Node X will be alwaysconnected to VA or VB by the blown fuse regardless of the device powerstatus. Signals GA and BL are constructed orthogonally to facilitate rowand column based decoding to construct these memory elements in anarray.

FIG. 8 shows two EEPROM non-volatile configuration circuits that can beused in this invention. Configuration circuit 650 in FIG. 6B can bereplaced with either of two EEPROM circuit shown in FIG. 8A and FIG. 8B.In FIG. 8A, node 840 is a floating gate. This is usually a poly-Siliconfilm isolated by an insulator all around. It is coupled to the sourceend of programming transistor 820 via a tunneling diode 830. Thetunneling diode is a thin dielectric capacitor between floating poly andsubstrate Silicon with high doping on either side. When a largeprogramming (or erase) voltage Vpp is applied across the thindielectric, a Fowler-Nordheim tunneling current flows through the oxide.The tunneling electrons move from electrical negative to electricalpositive voltage. Choosing the polarity of the applied voltage acrossthe tunneling dielectric, the direction of electron flow can bereversed. Multiple programming and erase cycles are possible for thesememory elements. As the tunneling currents are small, the highprogramming voltage (Vpp) can be generated on chip, and the programmingand erasure can be done while the chip is in a system. It is hencecalled in system programmable (ISP). An oxide or dielectric capacitor810 couples the floating gate (FG) 840 to a control gate (CG). Thecontrol gate CG can be a heavily doped Silicon substrate plate or asecond poly-Silicon plate above the floating poly. The dielectric can beoxide, nitride, ONO or any other insulating material. A voltage appliedto CG will be capacitively coupled to FG node 840. The coupling ratio isdesigned such that 60–80 percent of CG voltage will be coupled to FGnode 840. To program this memory element, a negative charge must betrapped on the FG 840. This is done by applying positive Vpp voltage onCG, ground voltage on PL and a sufficiently high (Vcc) voltage on RL. CGcouples a high positive voltage onto FG 840 creating a high voltage dropacross diode 830. Electrons move to the FG 840 to reduce this electricfield. When the memory device is returned to normal voltages, a netnegative voltage remains trapped on the FG 840. To erase the memoryelement, the electrons must be removed from the floating gate. This canbe done by UV light, but an electrical method is more easily adapted.The CG is grounded, a very high voltage (Vpp+more to prevent a thresholdvoltage drop across 820) is applied to RL, and a very high voltage (Vpp)is applied to PL. Now a low voltage is coupled to FG with a very highpositive voltage on the source side of device 820. Diode 830 tunnelingremoves electrons from FG. This removal continues beyond a chargeneutral state for the isolated FG. When the memory device is returned tonormal voltages, a net positive voltage remains trapped on the FG 840.Under normal operation RL is grounded to isolate the memory element fromthe programming path, and PL is grounded. A positive intermediatevoltage Vcg is applied to CG terminal. FG voltage is denoted S₀. UnderCG bias, S₀ signal levels are designed to activate pass-gate logiccorrectly. Configuration circuit in FIG. 8B is only different to that inFIG. 8A by the capacitor 851 used to induce S₀ voltage. This is usefulwhen S₀ output is applied to leaky pass-gates, or low level leakagenodes. As gate oxide thicknesses reach below 50 angstroms, thepass-gates leak due to direct tunneling.

These configuration circuits, and similarly constructed otherconfiguration circuits, can be used in programmable logic devices. Thosewith ordinary skill in the art may recognize other methods forconstructing configuration circuits to generate a valid S₀ output. Thepass-gate logic element is not affected by the choice of theconfiguration circuit.

SRAM memory technology has the advantage of not requiring a high voltageto configure memory. The SRAM based switch shown in FIG. 6B containingthe SRAM memory circuit shown in FIG. 6A utilizes 6 extra configurationtransistors, discounting the pass-gate 610, to provide theprogrammability. That is a significant overhead compared to applicationspecific and hard-wired gate array circuits where the point to pointconnection can be directly made with metal. Similarly other programmablememory elements capable of configuring pass-gate logic also carry a highSilicon foot print. A cheaper method of constructing a verticallyintegrated SRAM cell is described in incorporated by referenceapplication Ser. No. 10/413,810. In a preferred embodiment, theconfiguration circuit is built on thin-film semiconductor layers locatedvertically above the logic circuits. The SRAM memory element, athin-film transistor (TFT) CMOS latch as shown in FIG. 6A, comprises twolower performance back to back inverters formed on two semiconductorthin film layers, substantially different from a first semiconductorsingle crystal substrate layer and a gate poly layer used for logictransistor construction. This latch is stacked above the logic circuitsfor slow memory applications with no penalty on Silicon area and cost.This latch is adapted to receive power and ground voltages in additionto configuration signals. The two programming access transistors for theTFT latch are also formed on thin-film layers. Thus in FIG. 6B, all sixconfiguration transistors shown in 650 are constructed in TFT layers,vertically above the pass transistor 610. Transistor 610 is in theconducting path of the connection and needs to be a high performancesingle crystal Silicon transistor. This vertical integration makes iteconomically feasible to add an SRAM based configuration circuit at avery small cost overhead to create a programmable solution. Suchvertical integration can be extended to all other memory elements thatcan be vertically integrated above logic circuits.

A new kind of a programmable logic device utilizing thin-film transistorconfigurable circuits is disclosed in incorporated by referenceapplication Ser. No. 10/267,483, application Ser. No. 10/267,484 andapplication Ser. No. 10/267,511. The disclosures describe a programmablelogic device and an application specific device fabrication from thesame base Silicon die. The PLD is fabricated with a programmable RAMmodule, while the ASIC is fabricated with a conductive ROM pattern inlieu of the RAM. Both RAM module and ROM module provide identicalcontrol of logic circuits. For each set of RAM bit patterns, there is aunique ROM pattern to achieve the same logic functionality. The verticalintegration of the configuration circuit leads to a significant costreduction for the PLD, and the elimination of TFT memory for the ASICallows an additional cost reduction for the user. The TFT verticalmemory integration scheme is briefly described next.

FIG. 9 shows an implementation of vertically integrated circuits, wherethe configuration memory element is located above logic. The memoryelement can be any one of fuse links, anti-fuse capacitors, SRAM cells,DRAM cells, metal optional links, EPROM cells, EEPROM cells, flashcells, ferro-electric elements, electro-chemical elements, opticalelements and magnetic elements that lend to this implementation. SRAMmemory is used herein to illustrate the scheme and is not to be taken ina limiting sense. First, Silicon transistors 950 are deposited on asubstrate. A module layer of removable SRAM cells 952 are positionedabove the Silicon transistors 950, and a module layer of interconnectwiring or routing circuit 954 is formed above the removable memory cells952. To allow this replacement, the design adheres to a hierarchicallayout structure. As shown in FIG. 9, the SRAM cell module is sandwichedbetween the single crystal device layers below and the metal layersabove electrically connecting to both. It also provides throughconnections “A” for the lower device layers to upper metal layers. TheSRAM module contains no switching electrical signal routing inside themodule. All such routing is in the layers above and below. Most of theprogrammable element configuration signals run inside the module. Upperlayer connections to SRAM module “C” are minimized to Power, Ground andhigh drive data wires. Connections “B” between SRAM module and singlecrystal module only contain logic level signals and replaced later byVcc and Vss wires. Most of the replaceable programmable elements and itsconfiguration wiring is in the “replaceable module” while all thedevices and wiring for the end ASIC is outside the “replaceable module”.In other embodiments, the replaceable module could exist between twometal layers or as the top most module layer satisfying the same deviceand routing constraints. This description is equally applicable to anyother configuration memory element, and not limited to SRAM cells.

Fabrication of the IC also follows a modularized device formation.Formation of transistors 950 and routing 954 is by utilizing a standardlogic process flow used in the ASIC fabrication. Extra processing stepsused for memory element 952 formation are inserted into the logic flowafter circuit layer 950 is constructed. A full disclosure of thevertical integration of the TFT module using extra masks and extraprocessing is in the incorporated by reference applications listedabove.

During the ROM customization, the base die and the data in thoseremaining mask layers do not change making the logistics associated withchip manufacture simple. Removal of the SRAM module provides a low coststandard logic process for the final ASIC construction with the addedbenefit of a smaller die size. The design timing is unaffected by thismigration as lateral metal routing and Silicon transistors areuntouched. Software verification and the original FPGA designmethodology provide a guaranteed final ASIC solution to the user. A fulldisclosure of the ASIC migration from the original FPGA is in theincorporated by reference applications discussed above.

In FIG. 9, the third module layer is formed substantially above thefirst and second module layers, wherein interconnect and routing signalsare formed to connect the circuit blocks within the first and secondmodule layers. Alternatively, the third module layer can be formedsubstantially below the first and second module layer with interconnectand routing signals formed to connect the circuit blocks within thefirst and second module layers. Alternatively, the third and fourthmodule layers positioned above and below the second module layerrespectively, wherein the third and fourth module layers provideinterconnect and routing signals to connect the circuit blocks withinthe first and second module layers.

In yet another embodiment of a programmable multi-dimensionalsemiconductor device, a first module layer is fabricated having aplurality of circuit blocks formed on a first plane. The programmablemulti-dimensional semiconductor device also includes a second modulelayer formed on a second plane. A plurality of configuration circuits isthen formed in the second plane to store instructions to control aportion of the circuit blocks.

The fabrication of thin-film transistors to construct configurationcircuits is discussed next. A full disclosure is provided inincorporated by reference application Ser. No. 10/413,809. The followingterms used herein are acronyms associated with certain manufacturingprocesses. The acronyms and their abbreviations are as follows:

V_(T) Threshold voltage

LDN Lightly doped NMOS drain

LDP Lightly doped PMOS drain

LDD Lightly doped drain

RTA Rapid thermal annealing

Ni Nickel

Co Cobalt

Ti Titanium

TiN Titanium-Nitride

W Tungsten

S Source

D Drain

G Gate

ILD Inter layer dielectric

C1 Contact-1

M1 Metal-1

P1 Poly-1

P− Positive light dopant (Boron species, BF₂)

N− Negative light dopant (Phosphorous, Arsenic)

P+ Positive high dopant (Boron species, BF₂)

N+Negative high dopant (Phosphorous, Arsenic)

Gox Gate oxide

C2 Contact-2

LPCVD Low pressure chemical vapor deposition

CVD Chemical vapor deposition

ONO Oxide-nitride-oxide

LTO Low temperature oxide

A logic process is used to fabricate CMOS devices on a substrate layerfor the fabrication of logic circuits. These CMOS devices may be used tobuild AND gates, OR gates, inverters, adders, multipliers, memory andpass-gate based logic functions in an integrated circuit. A CMOSFET TFTmodule layer or a Complementary gated FET (CGated-FET) TFT module layermay be inserted to a logic process at a first contact mask to build asecond set of TFT MOSFET or Gated-FET devices. Configuration circuitryincluding RAM elements is build with these second set of transistors. Anexemplary logic process may include one or more following steps:

P-type substrate starting wafer

Shallow Trench isolation: Trench Etch, Trench Fill and CMP

Sacrificial oxide deposition

PMOS V_(T) mask & implant

NMOS V_(T) mask & implant

Pwell implant mask and implant through field

Nwell implant mask and implant through field

Dopant activation and anneal

Sacrificial oxide etch

Gate oxidation/Dual gate oxide option

Gate poly (GP) deposition

GP mask & etch

LDN mask & implant

LDP mask & implant

Spacer oxide deposition & spacer etch

N+ mask and NMOS N+ G, S, D implant

P+ mask and PMOS P+ G, S, D implant

Co deposition

RTA anneal—Co salicidation (S/D/G regions & interconnect)

Unreacted Co etch

ILD oxide deposition & CMP

FIG. 10 shows an exemplary process for fabricating a thin film MOSFETlatch in a second module layer. In one embodiment the process in FIG. 10forms the latch in a layer substantially above the substrate layer. Theprocessing sequence in FIG. 10.1 through FIG. 10.7 describes thephysical construction of a MOSFET device for storage circuits 650 shownin FIG. 6B. The process of FIG. 10 includes adding one or more followingsteps to the logic process after ILD oxide deposition & CMP step in thelogic process.

C1 mask & etch

W-Silicide plug fill & CMP

˜250 A poly P1 (amorphous poly-1) deposition

P1 mask & etch

Blanket Vtn P-implant (NMOS Vt)

Vtp mask & N-implant (PMOS Vt)

TFT Gox (70A PECVD) deposition

400A P2 (amorphous poly-2) deposition

P2 mask & etch

Blanket LDN NMOS N-tip implant

LDP mask and PMOS P-tip implant

Spacer LTO deposition

Spacer LTO etch to form spacers & expose P1

Blanket N+ implant (NMOS G/S/D & interconnect)

P+ mask & implant (PMOS G/S/D & interconnect)

Ni deposition

RTA salicidation and poly re-crystallization (G/S/D regions &interconnect)

Dopant activation anneal

Excess Ni etch

ILD oxide deposition & CMP

C2 mask & etch

W plug formation & CMP

M1 deposition and back end metallization

The TFT process technology consists of creating NMOS & PMOS poly-Silicontransistors. In the embodiment in FIG. 10, the module insertion is afterthe substrate device gate-poly etch and ILD film deposition. In otherembodiments the insertion point may be after M1 and ILD deposition,prior to VI mask, or between two metal definition steps.

After gate poly of regular transistors are patterned and etched, thepoly is salicided using Cobalt & RTA sequences. Then the ILD isdeposited, and polished by CMP techniques to a desired thickness. In theshown embodiment, the contact mask is split into two levels. The firstC1 mask contains all contacts that connect TFT latch outputs tosubstrate transistor pass-gates. This C1 mask is used to open and etchcontacts in the ILD film. Ti/TiN glue layer followed by W-Six plugs, Wplugs or Si plugs may be used to fill the plugs, then CMP polished toleave the fill material only in the contact holes. The choice of fillmaterial is based on the thermal requirements of the TFT module. Inanother embodiment, Ni is introduced into C1 to facilitatecrystallization of the poly Silicon deposited over the contacts. This Nimay be introduced as a thin layer after the Ti/TiN glue layer isdeposited, or after W is deposited just to fill the center of thecontact hole.

Then, a desired thickness of first P1 poly, amorphous or crystalline, isdeposited by LPCVD as shown in FIG. 10.1. The P1 thickness is between 50A and 1000 A, and preferably 250 A. This poly layer P1 is used for thechannel, source, and drain regions for both NMOS and PMOS TFT's. It ispatterned and etched to form the transistor body regions. In otherembodiments, P1 is used for contact pedestals. NMOS transistors areblanket implanted with P-doping, while the PMOS transistor regions aremask selected and implanted with N-doping. This is shown in FIG. 10.2.The implant doses and P1 thickness are optimized to get the requiredthreshold voltages for PMOS & NMOS devices under fully depletedtransistor operation, and maximize on/off device current ratio. Thepedestals implant type is irrelevant at this point. In anotherembodiment, the V_(T) implantation is done with a mask P-implantfollowed by masked N-implant. First doping can also be done in-situduring poly deposition or by blanket implant after poly is deposited.

Patterned and implanted P1 may be subjected to dopant activation andcrystallization. In one embodiment, an RTA cycle with Ni as seed in C1is used to activate & crystallize the poly before or after it ispatterned to near single crystal form. In a second embodiment, the gatedielectric is deposited, and buried contact mask is used to etch areaswhere P1 contacts P2 layer. Then, Ni is deposited and salicided with RTAcycle. All of the P1 in contact with Ni is salicided, while the restpoly is crystallized to near single crystal form. Then the un-reacted Niis etched away. In a third embodiment, amorphous poly is crystallizedprior to P1 patterning with an oxide cap, metal seed mask, Ni depositionand MILC (Metal-Induced-Lateral-Crystallization).

Then the TFT gate dielectric layer is deposited followed by P2 layerdeposition. The dielectric is deposited by PECVD techniques to a desiredthickness in the 30–200 A range, desirably 70 A thick. The gate may begrown thermally by using RTA. This gate material could be an oxide,nitride, oxynitride, ONO structure, or any other dielectric materialcombinations used as gate dielectric. The dielectric thickness isdetermined by the voltage level of the process. At this point anoptional buried contact mask (BC) may be used to open selected P1contact regions, etch the dielectric and expose P1 layer. BC could beused on P1 pedestals to form P1/P2 stacks over C1. In the P1 salicidedembodiment using Ni, the dielectric deposition and buried contact etchoccur before the crystallization. In the preferred embodiment, no BC isused.

Then second poly P2 layer, 100 A to 2000 A thick, preferably 400 A isdeposited as amorphous or crystalline poly-Silicon by LPCVD as shown inFIG. 10.3. P2 layer is defined into NMOS & PMOS gate regionsintersecting the P1 layer body regions, C1 pedestals if needed, andlocal interconnect lines and then etched. The P2 layer etching iscontinued until the dielectric oxide is exposed over P1 areas uncoveredby P2 (source, drain, P1 resistors). The source & drain P1 regionsorthogonal to P2 gate regions are now self aligned to P2 gate edges. TheS/D P2 regions may contact P1 via buried contacts. NMOS devices areblanket implanted with LDN N-dopant. Then PMOS devices are mask selectedand implanted with LDP P− dopant as shown in FIG. 10.4. The implantenergy ensures full dopant penetration through the residual oxide intothe S/D regions adjacent to P2 layers.

A spacer oxide is deposited over the LDD implanted P2 using LTO or PECVDtechniques. The oxide is etched to form spacers. The spacer etch leavesa residual oxide over P1 in a first embodiment, and completely removesoxide over exposed P1 in a second embodiment. The latter allows for P1salicidation at a subsequent step. Then NMOS devices & N+ polyinterconnects are blanket implanted with N+. The implant energy ensuresfull or partial dopant penetration into the 100 A residual oxide in theS/D regions adjacent to P2 layers. This doping gets to gate, drain &source of all NMOS devices and N+ interconnects. The P+ mask is used toselect PMOS devices and P+ interconnect, and implanted with P+ dopant asshown in FIG. 10.5. PMOS gate, drain & source regions receive the P+dopant. This N+/P+ implants can be done with N+ mask followed by P+mask. The V_(T) implanted P1 regions are now completely covered by P2layer and spacer regions, and form channel regions of NMOS & PMOStransistors.

After the P+/N+ implants, Nickel is deposited over P2 and salicided toform a low resistive refractory metal on exposed poly by RTA. Un-reactedNi is etched as shown in FIG. 10.6. This 100 A–500 A thick Ni-Salicideconnects the opposite doped poly-2 regions together providing lowresistive poly wires for data. In one embodiment, the residual gatedielectric left after the spacer prevents P1 layer salicidation. In asecond embodiment, as the residual oxide is removed over exposed P1after spacer-etch, P1 is salicided. The thickness of Ni deposition maybe used to control full or partial salicidation of P1 regions. Fullysalicided S/D regions up to spacer edge facilitate high drive currentdue to lower source and drain resistances.

An LTO film is deposited over P2 layer, and polished flat with CMP. Asecond contact mask C2 is used to open contacts into the TFT P2 and P1regions in addition to all other contacts to substrate transistors. Inthe shown embodiment, C1 contacts connecting latch outputs to substratetransistor gates require no C2 contacts. Contact plugs are filled withtungsten, CMP polished, and connected by metal as done in standardcontact metallization of IC's as shown in FIG. 10.7.

A TFT process sequence similar to that shown in FIG. 10 can be used tobuild complementary Gated-FET thin film devices. Compared with CMOSdevices, these are bulk conducting devices and work on the principles ofJFETs. A full disclosure of these devices is provided in incorporated byreference application Ser. No. 10/413,808. The process steps facilitatethe device doping differences between MOSFET and Gated-FET devices, andsimultaneous formation of complementary Gated-FET TFT devices. Adetailed description for this process was provided when describing FIG.10 earlier and is not repeated. An exemplary CGated-FET process sequencemay use one or more of the following steps:

C1 mask & etch

W-Silicide plug fill & CMP (optional Ni seed in W-plug)

˜300 A poly P1 (amorphous poly-1) deposition

Optional poly crystallization

P1 mask & etch

Blanket Vtn N− implant (Gated-NFET V_(T))

Vtp mask & P− implant (Gated-PFET V_(T))

TFT Gox (70 A PECVD) deposition

500A P2 (amorphous poly-2) deposition

Blanket P+ implant (Gated-NFET gate & interconnect)

N+ mask & implant (Gated-PFET gate & interconnect)

P2 mask & etch

Blanket LDN Gated-NFET N tip implant

LDP mask and Gated-PFET P tip implant

Spacer LTO deposition

Spacer LTO etch to form spacers & expose P1

Ni deposition

RTA salicidation and poly re-crystallization (exposed P1 and P2)

Fully salicidation of exposed P1 S/D regions

Dopant activation anneal

Excess Ni etch

ILD oxide deposition & CMP

C2 mask & etch

W plug formation & CMP

M1 deposition and back end metallization

As the discussions demonstrate, memory controlled pass transistor logicelements provide a powerful tool to make switches. The ensuing high costof memory can be drastically reduced by the 3-dimensional integration ofconfiguration elements and the replaceable modularity concept for saidmemory. These advances allow designing a LUT based macrocell with moreprogrammable bits to overcome the deficiencies associated with logicfitting in large LUT sizes. In one aspect, a cheaper memory elementallows use of more memory for programmability. That enhances the abilityto build large logic blocks utilizing multiple LUTs (i.e. course-grainadvantage) while maintaining smaller logic element type logic fitting(i.e. fine-grain advantage). Furthermore larger grains need lessconnectivity: neighboring cells and far-away cells. That furthersimplifies the interconnect structure. Larger grains benefit by largerLUT sizes, or a larger number of bigger LUTs in a logic block. In asecond aspect cheaper memory allows LUT partitioning that canefficiently utilize Silicon by fitting large and small logic pieces intoa single large LUT. Such LUTs can improve Silicon utilization comparedto FIG. 4. A new programmable LUT macrocell circuit utilizing themanufacturing methods shown so far is discussed next. Larger LUTintegration is discussed by Wittig et al. U.S. Pat. No. 6,208,163,Agrawal et al. US 2002/0186044, Sueyoshi et al. US 2003/0001615 and Pughet al. US 2003/0085733. They do not show the need, a method and thevalue in using programmable bits to provide multiple smaller LUTpartitioning inside a single larger LUT for FPGA designs.

A one input LUT (1LUT) according to current teaching is shown in FIG.11A. The LUT is comprised of input A driving pass-gate 1101. Inputcompliment A′ drives pass-gate 1102. Cross-circled elements 1111, 1112 &1113 represent memory bits in a configurable memory circuit. An SRAMbased memory circuit described earlier is shown in FIG. 6. Such a memorycircuit provides complimentary outputs S₀ & S₀′ to control on-offbehavior of pass-gates 1101–1106. The LUT values are selected byprogrammable bit such as 1111 in one of two configurations. When thememory bit is programmed to a logic one, the bit 1111 outputs a logicone S₀ on the right hand side branch and logic zero S₀′ on the left handbranch. When the memory bit is programmed to a logic zero, the bit 1111outputs a logic zero S₀ on the right hand side branch and logic one S₀′on the left hand branch. This allows selecting I₁, I₂ pair as LUT valuesby setting memory bit 1111 to zero, or selecting values stored inregister 1112, 1113 pair as LUT values by setting memory bit 1111 toone. The inputs I₁ and I₂ are also driven by buffers that are not shownin FIG. 11A. Memory bits 1111, 1112 & 1113 are constructed in athin-film module and are vertically integrated. TFT SRAM 1112 and 1113drive inverters constructed in substrate Silicon or pass-gates couplingVcc & Vss to provide necessary LUT value drive currents. All TFT memorycircuits allow the user to change stored data as desired. Theconfiguration circuits including memory is constructed over thepass-gate logic circuits and consumes no Silicon area and cost. Whenselected, the registers 1112 & 1113 can be independently set to logicstates one or zero by the user, and becomes identical to the 1LUT shownin FIG. 3A. Once the desired memory pattern is identified by the user,TFT elements 1111, 1112 & 1113 can be replaced by hard-wires connectedto Vcc or Vss to achieve identical logic functionality. As the timingpath is restricted to signal propagation in wires and pass-gates, thereis no change in timing with this conversion. As the fabrication processis simplified by eliminating TFT memory processing, the end product ischeaper to fabricate and more reliable for the user.

Two Embodiments of block diagrams of the LUT shown in FIG. 11A are shownin FIG. 11C and FIG. 11D. Referring to FIG. 11C, a programmable look uptable (LUT) circuit 1138 for an integrated circuit, comprises: one ormore secondary inputs 1132; and one or more configurable logic states1134; and two or more LUT values 1135, 1136; and a programmable means1133 to select a LUT value from a secondary input 1132 or a configurablelogic state 1134. Referring to FIG. 11D, the circuit 1148 furthercomprises: a LUT output 1147; and M primary inputs such as 1141, where Mis an integer value greater than or equal to one, each said M inputsreceived in true and compliment logic levels; and 2^(M) LUT values suchas 1145 & 1146, each said LUT values comprising a configurable logicstate or a secondary input, wherein any given combination of said Mprimary input signal levels couples one of said LUT values to said LUToutput.

An equivalent MUX representation for FIG. 11A is shown in FIG. 11B. TheLUT values are chosen from two 3-input MUXs 1151 and 1152 with 3programmable bits, wherein the gate construction is as in FIG. 11A, andthe block diagram is as in FIG. 11D.

A second embodiment of a programmable 1LUT according to this teaching isshown in FIG. 12A. This 1LUT utilizes 4-programmable memory bits 1211,1212, 1213 and 1214, and otherwise identical to 1LUT in FIG. 11A. Having4 programmable bits allows the user to select the upper half of 1LUTindependent of the lower half. For example, bit 1211 can be configuredto select I₁ as a LUT value for A input, and bit 1214 can be configuredto select register 1213 as the LUT value for A′ input. This flexibilityin a LUT macrocell is extremely useful to reduce Silicon wastage as willbe shown later. Another embodiment of the programmable macro-cellaccording to these teachings utilizing 4-programmable bits is shown inFIG. 12B. This has two 4:1 MUXs 1351 and 1352 that are configured by 2bits each for each LUT value. Each 4:1 MUX is identical to the MUX shownin FIG. 2C. LUT value for input A is programmed from I₁, I₂, 0 & 1,while LUT value for input A′ is programmed from I₃, I₄, 0 & 1. This 1LUTmacro-cell allows the user to select which inputs needs to couple fromprevious to next LUT stage. When I₁=I₃=B and I₂=I₄=B′, FIG. 12B becomesa 2-input LUT. Memory circuits for FIG. 12 are also constructed in TFTlayers to occupy no extra Silicon area.

A third embodiment of a programmable 1LUT according to this teaching isshown in FIG. 13A. This 1LUT also utilizes 4-programmable memory bits1311, 1312, 1313 and 1314, but provides an option for inputs I₁ and I₂to by-pass the 1LUT. Otherwise, FIG. 13A is identical to 1LUT in FIG.12A. Bit 1311 polarity controls both logic state 1312 selection andinput I₁ by-pass. When LUT values are chosen to be logic states from1312 & 1313, the inputs 1321 & 1322 are by-passed to registers not shownin the FIG. 13A. The circuit shown in FIG. 13A has a programmable method1311 further comprising a means of providing said secondary input 1321as an output when said configurable logic state 1312 is selected as aLUT value. Secondary input 1312 is provided as an output via the by-passpass-gate 1308. Having 4 programmable bits allows the user to select theupper half of 1LUT independent of the lower half. For example, bit 1311can be configured to select I₁ as a LUT value for A input and disable I₁by-pass pass-gate 1308. Bit 1314 can be configured to select register1313 output as the LUT value for A′ input and shunt I₂ input to anoutput register through pass-gate 1303. This flexibility in a LUTmacrocell is also useful to reduce Silicon wastage as will be shownlater. Yet another embodiment of the programmable macro-cell accordingto these teachings utilizing 6-programmable bits is shown in FIG. 13B.This has two 8:1 MUXs 1351 and 1352 that are configured by 3 bits each.Each 8:1 MUX is a conventional MUX similar to the 4:1 MUX shown in FIG.2C. Upper half of 1LUT and lower half of 1LUT are independentlyprogrammed to one of eight choices for that LUT value. Apart from 0 and1, the remaining 6 LUT value choices need not be identical. This LUTmacro-cell allows the user to select multiple inputs in a LUT structureto perform a logic function of two variables. Memory circuits for FIG.13 are constructed in TFT layers.

A 2-input LUT construction from programmable 1LUTs is shown in FIG. 14.The 2LUT has 4 LUT values in registers 1421, 1422, 1423 and 1424. TheseLUT values are controlled by common input B on pass-gates 1401, 1402,1403 and 1404. The outputs from this first stage are fed to aprogrammable 1LUT similar to the one discussed in FIG. 13A. Fourprogrammable registers 1425, 1426, 1427 and 1428 control the secondstage 1LUT providing the capability of combining the 2 LUTs or usingthem independently.

A 3-input LUT (3LUT) according to present invention is shown in FIG. 15.Two conventional 2LUTs 1501 and 1502 are fed to a programmable 1LUTdiscussed in FIG. 13A. This LUT macrocell can be configured to performtwo independent 2LUT functions and one 1LUT function. The 2LUT outputscan by-pass the 1LUT and feed registers not shown in FIG. 15. LUTmacrocell can also perform one 3LUT function when C & E are made commonand B & D are made common. In addition, the LUT macrocell can alsoperform a 3LUT (when the 3LUT function has half of the truth tableentries as zero or one) plus a 2LUT. It can also perform some 4-inputand 5-input variable functions. These divisions in logic allow improvedlogic fitting into LUT macrocells.

A 4-input LUT (4LUT) according to present invention is shown in FIG. 16Aand FIG. 16B. In FIG. 16A, four conventional 2LUTs 1601–1604 are fed toa programmable 2LUT 1605. The 2LUT 1605 is constructed with 2programmable 1LUTs discussed in FIG. 13A. This LUT macrocell can beconfigured to perform a wide variety of logic functions. It can performfive independent 2LUT functions, and all 2LUT outputs can be fed toregisters (not shown). This is done by programming 2LUT 1605 to fullindependent mode by selecting all configurable states (such as 1613 &1614) as LUT values. It can also perform one 4LUT function when firststage inputs (D, F, H, K) are made common and second stage inputs (C, E,G, J) are made common. There may be programmable switches to make thesecommon inputs. When the 4LUT function has rows or columns in the truthtable entries as zero or one, a LUT value is chosen in 2LUT 1605 to savea full 2LUT in a prior stage. Hence the LUT macrocell can also performsa 4LUT plus one or more 2LUTs to enhance logic density. It can alsoperform some 5-input, 6-input, up to 10-input variable functions. TheLUT inputs are selected from a group of external inputs by programmableMUXs not shown in the diagram. These divisions in logic allow improvedlogic fitting into LUT macrocell based architectures. Compared topercentage logic overhead for 1LUT 1503 in FIG. 15, the percentageoverhead required for the added flexibility in 2LUT 1605 is lower inFIG. 16A.

Referring to FIG. 16A, A programmable look up table circuit 1605 for anintegrated circuit, comprises: M primary inputs (such as A & B), whereinM is an integer value greater than or equal to one, and each said Minputs received in true and compliment logic levels; and 2^(M) secondaryinputs (such as 1611, 1612); and 2^(M) configurable logic states (suchas 1613, 1614), each said state comprising a logic zero and a logic one;and 2^(M) LUT values; and a programmable means to select each of saidLUT values from a secondary input (such as 1611) or a configurable logicstate (such as 1613). In circuit 1605, each of said secondary inputs(such as 1611) is further comprised of an output of a previous K-LUTcircuit (such as 1601), said K-LUT circuit comprising: a LUT output(same as 1611); and K inputs (such as C & D), wherein K is an integervalue greater than or equal to one, and each said K inputs received intrue and compliment logic levels; and 2^(K) LUT values (such ascrossed-circle latch outputs in 1601), each said LUT values comprisingtwo configurable logic states.

Referring to FIG. 16A, a larger N-LUT is constructed with smaller K-LUTs(such as 1601–1605). Each smaller K-LUT is further constructed as oneof: 1LUT, 2LUT, 3LUT up to (N−1)-LUT smaller LUTs. In FIG. 16A, K isequal to 2. The N-LUT is constructed as a K-LUT tree, staged withK-LUTs, where 2^(K) outputs from a first stage feed as LUT values toeach of next stage. Each K-LUT has 2^(K) LUT values and K inputs. Thereis a 2^(K) reduction in the number of K-LUTs from one stage to the next.The last K-LUT has only one output. Each K-LUT (such as 1601) in turn iscomprised of one or more 1LUTs arranged in one or more stages. The K-LUTis also constructed as a 1LUT tree, staged with 1LUTs, where two outputsof a first stage feed as LUT values to next stage. A secondary K-LUTstage (such as 1605) provides programmability in connecting K-LUTs (from1601–1604) to form an N-LUT tree. K-LUTs 1601–1604 outputs can by-passK-LUT 1605 to registers. By programming the by-pass option, all K-LUTscan be used independently. A first stage in a secondary K-LUT 1605comprises 1LUTs having two LUT values that can be configured to be oneof two options: programmable logic states (such as 1613 output), or twoprevious LUT outputs (such as 1611). Except the first stage, everysubsequent secondary LUT stages in the N-LUT may have K-LUTs comprisinga first stage with this programmable capability. When LUT values areconfigured as logic states, the N-LUT may compute (2^(N)−1)/(2^(K)−1)independent smaller K-LUT functions. When all secondary LUT values areconfigured as outputs from previous LUTs, and the K-inputs in each stageis made common to all K-LUTs in that stage, the K-LUT may be used toconstruct one N-LUT logic function. When all the K-LUT inputs are notmade common to all the K-LUTs in that stage, a logic function with morethan N-inputs may fit into an N-LUT tree. This hierarchical K-LUTsarrangement is called a LUT macrocell circuit. The LUT macrocell provideprogrammability to combine multiple smaller LUTs to one larger LUT, orimplement logic in smaller LUT form.

The circuit in FIG. 16B is only different to that in FIG. 16A on themethod of choosing inputs to programmable 2LUT 1625. Both A and B inputshave the capability of being selected from external inputs V, X, Y & Z,or prior LUT outputs I₁, I₂, I₃ & I₄. The programmable look up table(LUT) macro-cell circuit for an integrated circuit in FIG. 16B,comprises: a plurality of LUT devices 1621–1625; each said LUT devicehaving an output (such as I₁–I₄, F), at least one input (such as A–K),and at least two LUT values; and a programmable means (such as MUX 1651)of selecting inputs to at least one of said LUT devices from one or moreother LUT device outputs and external inputs; and a programmable meansof selecting LUT values to at least one of said LUT device (such as1625) from one or more other LUT device outputs and configurable logicstates. The crossed-circles show memory bits that need programming tocustomize the LUT functions. The Silicon consumption for SRAM cells isreduced as demonstrated by the incorporated references.

A programmable macro look up table (macro-LUT) circuit in FIG. 16B foran integrated circuit, comprises: a plurality of LUT circuits(1621–1625), each of said LUT circuits comprising a LUT output, at leastone LUT input, and at least two LUT values; and a programmable means(such as 1651) of selecting LUT inputs to at least one of said LUTcircuits from one or more other LUT circuit outputs and external inputs,and selecting LUT values to at least one of said LUT circuits (such as1625) from one or more other LUT circuit outputs and configurable logicstates, said programmable means further comprised of two selectablemanufacturing configurations, wherein: in a first selectableconfiguration, a random access memory circuit (RAM) is formed, saidmemory circuit further comprising configurable thin-film memoryelements; in a second selectable configuration, a hard-wire read onlymemory circuit (ROM) is formed in lieu of said RAM, said ROM duplicatingone RAM pattern in the first selectable option.

A 5-input LUT (5LUT) can be easily constructed with the method presentedin FIG. 16. The four circuits 1601–1604 can be replaced by fourconventional 3LUTs. The four outputs can be fed as shown in FIG. 16 intothe programmable 2LUT. Similarly a 6LUT macrocell can be constructed byconstructing four conventional 4LUTs in the first stage in FIG. 16. Theoutputs from 4LUTs are then fed to the programmable 2LUT as shown inFIG. 16. Two programmable 3LUT versions are shown in FIG. 17A and FIG.17B. In FIG. 17A, six 1LUTs as discussed in FIG. 13A are combined asshown. In FIG. 17B, seven 1LUTs as discussed in FIG. 13A are combined intwo stages as shown. A 6LUT macrocell can be constructed by combiningsix conventional 3LUTs with either of the two programmable 3LUTs shownin FIG. 17A and FIG. 17B. A programmable look up table (LUT) circuit inFIG. 17A for an integrated circuit, comprises: N primary inputs (such asA, B, C), wherein N is an integer value greater than or equal to one,and each said N inputs received in true and compliment logic levels; and2^(N) secondary inputs (such as I₁–I₈); and 2^(N) LUT values, each saidLUT values comprising a programmable method to select between one ofsaid secondary inputs (such as I₁–I₈) or a configurable logic state(such as one of 1701–1708).

The efficiency of these LUT macrocells in Silicon utilization can bedemonstrated with the 4-variable truth table and the logic functionshown in FIG. 18A. It realizes a function that lends to truth tablelogic reduction. A 1LUT gate realization of the function is shown inFIG. 18B. It uses only four 1LUTs. The same function is ported to a 4LUTshown in FIG. 18C. There are 15 equivalent 1LUTs in the 4LUT, and allare required to implement the function. The 4LUT is seen to occupy 3.75×more pass-gate Silicon in this example compared to an idealimplementation shown in FIG. 18B (without counting the programmablememory bits required to set the LUT values). If we use the 4LUTmacro-cell shown in FIG. 16 which provides 2LUT divisibility, thisfunction can be implemented as shown in FIG. 18D. The bit polarityrequired to achieve the desired functionality are shown next to each bitin FIG. 18D. That allows two 2LUTs 1803 and 1804 to be used for other2-input logic functions. Those outputs can be taken out to registers viathe by-pass circuitry. The macrocell shown in FIG. 16 can be partitionedinto 2LUTs by design and used as five 2LUT blocks. It uses an equivalentof 21 1LUT gates, compared to 15 for the 4LUT in FIG. 18C. Column-4 inFIG. 4 shows that 4LUT on the average is only 36% efficient compared to2LUTs at fitting logic. Accounting for 21/15 inefficiency for the largerSi foot-print in the 4LUT macrocell in FIG. 16, it is still ˜2× moreefficient at fitting an average logic design in 2LUT pieces.

Each of the circuits described in FIG. 11 thru FIG. 17 provides aprogrammable means to configure the LUT macrocell. Said programmablemeans comprises a memory circuit fabricated with two selectablemanufacturing configurations. In a first selectable configuration a RAMcircuit is formed to provide said LUT user re-programmability. In asecond selectable configuration a ROM circuit is formed in lieu of onespecific RAM pattern to provide identical LUT programmability.

New programmable LUT circuits are described for use in large and finegeometry FPGA devices. As the logic density increases, there is a needto add more LUTs into a logic block, and increase the LUT size. Bothinhibit the efficiency of Silicon utilization when porting logicsynthesized to an ASIC flow. Compared to 2LUT based logic blocks, 4LUTsare seen to be only 36% efficient, while 7LUTs are only 7% efficient.The new LUT circuits disclosed herein make use of additionalprogrammable elements inside the large LUT structure, enablingsub-division of LUTs. A complex design can be fitted as a single largerlogic LUT or as many smaller logic LUT pieces: both maximizing theSilicon utilization. A 2LUT divisible 4LUT macro-cell shown in FIG. 16Aprovides a 2× improvement in logic packing compared to hard-wired 4LUTlogic elements. The increased memory content is justified by a3-dimentional thin-film transistor module integration that allows allconfiguration circuits to be built vertically above logic circuits.These memory circuits contain memory elements that control pass-gatesconstructed in substrate Silicon. The TFT layers are fabricated above acontact layer in a removable module, facilitating a novel method toremove completely from the process. Configuration circuits are mapped toa hard-wire metal links to provide the identical functionality in thelatter. Once the programming pattern is finalized with the thin-filmmodule, and the device is tested and verified for performance, the TFTcells can be eliminated by hard-wire connections. Such conversions allowthe user a lower cost and more reliable end product. These productsoffer an enormous advantage in lowering NRE costs and improving TTS inthe ASIC design methodology in the industry.

Although an illustrative embodiment of the present invention, andvarious modifications thereof, have been described in detail herein withreference to the accompanying drawings, it is to be understood that theinvention is not limited to this precise embodiment and the describedmodifications, and that various changes and further modifications may beeffected therein by one skilled in the art without departing from thescope or spirit of the invention as defined in the appended claims.

1. A programmable look up table (LUT) structure for an integratedcircuit, comprising a first stage and one or more intermediate stagesand a last stage, wherein at least one of said intermediate stages orthe last stage further comprises: a primary input received in true andcompliment logic levels, and an output; and two LUT values, said primaryinput coupling one of said LUT values to said output, wherein at leastone of said LUT values further comprises: a secondary input and aconfigurable data value; and a programmable means to select either thesecondary input or the data value as the LUT value.
 2. The structure ofclaim 1, wherein said secondary input is an output of a prior stage inthe LUT structure.
 3. The structure of claim 1, wherein saidconfigurable data value is an output of a configurable memory element.4. The structure of claim 1, wherein said configurable data value isfurther programmable to a logic zero or a logic one by said programmablemeans.
 5. The structure of claim 3, wherein said programmable meanscomprises a configuration circuit further comprised of one or moreconfigurable memory elements.
 6. The structure of claim 5, wherein saidconfigurable memory element is one of user configurable RAM element or amask configurable ROM element.
 7. The structure of claim 5, wherein saidmemory element is one of fuse link, anti-fuse capacitor, laser-fuse,SRAM cell, DRAM cell, metal optional link, EPROM cell, EEPROM cell,flash cell, ferro-electric element, optical element, electro-chemicalelement, electrolytic element, Carbon nano-tube, electro-mechanicalelement, electro-magnetic element and magnetic element.
 8. Aprogrammable macro look up table (LUT) circuit for an integratedcircuit, comprising: one or more first LUT circuits comprised of: a LUToutput; and M primary inputs, where M is an integer value greater thanor equal to two, each said M inputs received in true and complimentlogic levels; and 2^(M) LUT values, each said LUT values comprising aconfigurable data value at logic zero or logic one, wherein any givencombination of said M primary input signal levels couples one of saidLUT values to said LUT output; and a second LUT circuits comprised of: aLUT output; and at least one primary input, said input received in trueand compliment logic levels; and at least two LUT values, wherein atleast one LUT value is further comprised of: an output of one of saidfirst LUT circuits; and a configurable data value at logic zero or logicone; and a programmable means to select said first LUT circuit output orconfigurable data value as the LUT value; wherein, said at least oneprimary input signal couples one of the LUT values to said LUT output.9. The circuit of claim 8, wherein said configurable data value isgenerated by an output of a configurable memory element.
 10. The circuitof claim 8, wherein said configurable data value is further programmableto said logic zero or logic one by said programmable means.
 11. Thecircuit of claim 8, wherein an (M+1) input generic LUT circuit isimplemented by: two first LUT circuits, each comprised of M inputs; andone second LUT circuit with one primary input; wherein, the outputs ofthe two first LUT circuits are programmed as the two LUT values for thesecond LUT circuit.
 12. The circuit of claim 8, wherein an (M+1) inputspecial LUT circuit is implemented by: one first LUT circuit comprisedof M inputs; and one configurable data value; and one second LUT circuitwith one primary input; wherein, the output of the first LUT circuit andthe configurable data value are programmed as the two LUT values for thesecond LUT circuit.
 13. The circuit of claim 10, wherein saidprogrammable means further comprises a configuration circuit comprisedof one or more configurable memory elements.
 14. The circuit of claim13, wherein said configurable memory element is one of user configurablerandom access memory (RAM) element or a mask configurable read onlymemory (ROM) element.
 15. The circuit of claim 13, wherein said memoryelement is one of fuse link, anti-fuse capacitor, laser-fuse, SRAM cell,DRAM cell, metal optional link, EPROM cell, EEPROM cell, flash cell,ferro-electric element, optical element, electro-chemical element,electrolytic element, Carbon nano-tube, electro-mechanical element,electro-magnetic element and magnetic element.
 16. The circuit of claim13, further comprising a silicon substrate layer, wherein the first andsecond LUT circuits further comprises a transistor constructed on saidsubstrate layer, and wherein said configuration memory element isconstructed above the silicon substrate layer.
 17. The circuit of claim13, further comprising a first metal layer, wherein the first and secondLUT circuits further comprises a transistor constructed on a thin filmlayer, and wherein said transistor and the configuration memory elementare constructed above the first metal layer.
 18. A programmable look uptable (LUT) structure for an integrated circuit, comprising a firststage and one or more intermediate stages and a last stage, wherein thelast stage further comprises: a primary input, said input received intrue and compliment logic levels; and a first and a second output; and afirst and a second LUT value, each said LUT values further comprised of:a secondary input, said input generated by an output from a prior LUTstage; and a configurable data value at logic zero or logic one; and aprogrammable means to couple the secondary input or the data value tosaid LUT value; wherein, said primary input couples one of the LUTvalues to said first output; and wherein said programmable means furtherprovides the means to couple one or both of: the secondary input of thefirst LUT value to said second output; the secondary input of the secondLUT value to said second output.
 19. The structure of claim 18, furthercomprising a third output, wherein said programmable means furtherprovides the means to couple one or both of: the secondary input of thefirst LUT value to said third output; the secondary input of the secondLUT value to said third output.
 20. The structure of claim 18, furthercomprising a configuration circuit, wherein the configuration circuitcomprises a random access memory (RAM) element or a read only memory(ROM) element.